jump scare

Short definition from Wiktionary

Noun

1. 1
   The technique, typically used in horror films and video games, of having something occur suddenly and without warning to frighten the audience.
2. 2
   The sudden arrival or advent of something unpleasant.
3. 3
   A fright caused by this.
4. 4
   Alternative spelling of jump scare.

Verb

1. 1
   To frighten (a film audience or video game player, etc.) by means of something that happens suddenly without warning.